Method of producing fine wood flour



Patented Dec. 12, 1944 METHOD OF PRODUCIN G FINE WOOD Andrew W. Kassay, Emil H. Balz, and William D.

W'afliams, Toledo. Ohio. assignors, by mesne-assigmnents, to Libbey-Owens-Ford Glass Company. a corporation or 01110 No Application dune Serial No. 4456M The invention relates to a method of produc-' .(Ei. 33-r-94) ing fine wood flour that is useful as a filler for I synthetic resin compositions, such as moldin compounds and adhesives, and in other products, such as insecticides, for which fine wood flour is now used.

Wood flour is diflicult to reduce to a fine powder because of the fibrous nature of wood. The cellulose fibers of which'wood is made are tough and flexible, and the individual fibers are believed to have a higher tensile strength than steel. For many applications extremely fine wood flour is desirable, and such wood, flour is dilicult to produce by grinding because of the toughness of the cellulose fibers.

It has been proposed to treat wood with acid in order to reduce its toughness and thus render it brittle enough m be finely ground. However, the treatment of wood with acid does not produce satisfactory results because the acid causes degradation of the cellulose into soluble products. The resulting material is not suitable for use as an inert filler becauseit contains a conslderable proportion of degradation products of cellulose that are soluble in water.

It has also been proposed to impregnate wood with formaldehyde in order to render it brittle enough to be ground to a fine powder. However, formaldehyde has no actual embrittling action upon wood, and any eflect obtained by treating wood with; a formaldehyde solution is believed to arise from acid present in the solution and to be the same efle'ct as obtained by merely treating wood with acid.

The principal object of the invention is to provide a method of treating cellulose to render it brittle enough to beground to a fine powder without degradation or other impairment of its quality. More specific objects and advantages are apparent from the description which discloses and illustrates the invention and is not intended to impose limitations upon the claims.

In accordance with the invention, wood is impregnated with a solution of urea and is then dried and ground. The impregnation of the wood with the urea renders it brittle. Wood that has been so impregnated is easily ground and may be reduced to particles of minimum size by grinding in a ball mill in about one-half the time required to reduce unimpregnated wood to particles of minimum size in a ball mill. Other forms of cellulose may be used instead of wood.

The impregnation may be carried out by means of a solution of urea in any desired solvent since the solvent is subsequentl evaporated during the drying operation. The impregnation can be carried out more effectively if the wood is reduced to a relatively fine condition before the impregnating operation. Thus, it is preferable to pass chips, shavings and saw-dust through a rotary cutter to produce fine saw-dust, and then to pass the fine saw-dust through an attrition mill to convert it into coarse wood flour. The coarse wood flour is then impregnated with the solution of urea.

The amount of urea used is preferably about 5 per cent of the weight of wood. If the amount of urea used to impregnate the wood is substantially less than 5 per cent of the weight of the wood, satisfactory results may not be obtained. The amount of urea may be increased above 5 per cent of the weight of the wood, but such an increase does not produce a substantial improvement in results.v

The most satisfactory apparatus for grinding the particles of impregnated wood is a ball mill or a mill having a similar grinding action, such as a tube mill, or other tumbling impact mill.

in the following example, the wood may be replaced by walnut shell, etc. A mixture of chips, shavings and saw-dust is passed through a. rotary cutter to convert the material to a fine sawdust, and the fine saw-dust is then passed through an attrition mill which reduces it to the form' t of coarse wood flour. 20 parts of the resulting coarse wood flour is stirred with a solution of 1 part of urea in a sufllcient quantity of water to produce a paste of the desired consistency. The impregnated wood fiour so prepared is then spread out and dried. The drying may be carried out by mere exposure to the atmosphere, but it is expedited by circulation of the air and by heating the air to a temperature below the decomposition temperature of the impregnated wood flour. The dry product is then ground for several hours in a ball mill or tube mill, until a.

fragments with sufflcient solution of urea. to deposit about 5 per cent by weight of urea therein, drying, and grinding the dry urea-impregmated cellulose at ordinary temperatures.

3. Amethod of producing fine wood flour that comprises impregnating wood flour with sumcient solution of urea. to deposit not substantially less than about 5 per'cent by weight of urea therein, drying, and grinding the dry urea-impregnated cellulose at ordinary temperatures in a tumbling impact xnill. V

EMIL H. BALZ. ANDREW! W. KASSAY.

WILLIAM D. WILLIAMS, 

